D.C. Offset Estimation

ABSTRACT

A combination of a phase shifter, a measurement receiver, and an offset estimator enable the d.c. offset in the transmit path of a quadrature transmitter to be distinguished from the d.c. offset in the measurement receiver. The measurement receiver performs a first measurement on the transmit path output with a “normal” phase shift of 0 degrees and 90 degrees for in-phase (I) and quadrature (Q) components, and a second measurement with a “special” phase-shift of 180 degrees and 270 degrees for the I and Q components, respectively

TECHNICAL FIELD

This invention relates to electronic communication systems and moreparticularly to signal offset compensation in such systems.

BACKGROUND

Many current electronic communication systems use quadrature modulationschemes, which use in-phase (I) and quadrature (Q) signal components,and do not have constant envelopes. Examples of such communicationsystems are cellular radio telephone systems that use wideband codedivision multiple access (WCDMA), orthogonal frequency division multipleaccess (OFDMA), and their variants. Thus, part of the communicatedinformation is encoded in the amplitude (envelope) of the transmittedsignal and part is encoded in the phase of the transmitted signal.

To avoid distorting the communicated information, the power amplifier(PA) and various other components of the radio transmitter have to belinear, which is to say for example that the functional relationshipbetween the output power of the PA and the input power of the PA is astraight line for all possible power levels. In addition, the phaseshift of the input signal for example through the PA has to be constantfor all possible power levels.

Departures from amplitude linearity and constant phase introducedistortion into the communicated signal, such as spectral broadeningthat can disturb adjacent channels. Amplitude/phase distortion (vectordistortion) in the transmitter can also increase the bit error rate(BER) of the communication system, e.g., degrading the audio quality ofa voice call or reducing the speed of an internet connection.

In general, the likelihood of proper performance can be increased byincluding in the transmitter a measurement receiver (MRX) that samplesthe transmitted signal and generates a compensation signal is fed backto the modulator, PA, and/or other transmitter components to correct thetransmitter output signal. Such an arrangement 100 is depicted in FIG.1, which shows an antenna 102, a coupler 104, an amplifier 106, aquadrature modulator 108, and an MRX 110. The amplifier 106 andmodulator 108 can be considered the “transmit path” of the arrangement100, which it will be understood typically includes oscillators andother components not shown. As seen in the figure, the MRX 110 samplesthe transmitted signal generated by the transmit path through theoperation of the coupler 104 and provides a compensation signal to themodulator 108.

The MRX 110 can be used for several purposes, one of which ismeasurement, or more generally estimation, of the direct-current (d.c.)offset between I and Q components in the amplifier 106 and quadraturemodulator 108. To achieve that purpose correctly, the I/O d.c. offset ofthe MRX itself typically must be negligible (ideally, it should be zero)or at least well known. Otherwise, the I/O d.c. offset of the transmitpath will generally not be correctly estimated.

European Patent Application Publication No. EP 1 835 626 A1 by Ishikawaet al. describes a d.c. offset correction value estimating unit thatestimates a d.c. offset correction value based on a transmit signal thatis produced by a quadrature modulator. A signal level detecting unitdetects the signal level of an input signal, a weight factor calculatingunit computes a weight factor for the d.c. offset correction value inaccordance with the signal level, and a weighting unit assigns a weightto the d.c. offset correction value in accordance with the weightfactor. A d.c. offset in the transmit signal is compensated by using thethus weighted d.c. offset correction value.

U.S. Patent Application Publication No. US 2007/0092023 by Kang et al.describes a method for self-calibrating mismatch and d.c. offset in amobile transceiver. The transceiver's transmitter is used as a signalgenerator and the transceiver's receiver is used to measure a responsecharacteristic. A baseband processor calibrates the mismatch and thed.c. offset for the receiving and transmitting sides using a test signalreceived from the transmitter.

U.S. Pat. No. 7,266,359 to Chen et al. describes a method for removingd.c. interference from a signal received by a communication receiverthat removes d.c. offsets induced by the receiver and the transmitter.The method includes removing an estimated d.c. offset from a receivedsignal, correcting a frequency shift in the received signal, estimatinga second d.c. offset signal induced by the transmitter, and removing the

estimated second d.c. offset from the received signal. The receiver d.c.offset is estimated and removed before performing a timing carrieroffset correction using Barker code manipulation to remove receiver d.c.offset and to sum all Barker chips after effectively multiplying Barkercodes to correlate to a Barker sequence unaffected by the receiver d.c.offset signal.

U.S. Patent Application Publication No. US 2008/0063113 by Gao et al.describes a method of correcting d.c. offset errors in a transmitterhaving an OFDMA-based quadrature modulator. A compensator before themodulator compensates the d.c. offset and is updated with estimated d.c.offset values obtained by performing a discrete Fourier transform in thedigital baseband domain while sending a pair of orthogonal test tones tothe modulator's inputs.

U.S. Patent Application Publication No. US 2009/0041161 by Jian et al.describes a d.c. offset estimation in an OFDMA system that includes acarrier frequency offset estimator receiving an input signal andestimating a carrier frequency offset value, a symbol timing recoveryunit providing a symbol boundary of the input signal, and a d.c. offsetestimator estimating a d.c. offset value based on the input signal, thecarrier frequency offset value, and the symbol boundary.

Despite those and other previous attempts, the problem of d.c. offsetestimation remains difficult to solve, and the requirements on an MRXremain difficult to meet.

SUMMARY

This invention enables the I/O d.c. offset in the transmit path to beestimated without prior knowledge of the I/O d.c. offset of the MRXitself. Moreover, this invention enables both the I/O d.c. offset of thetransmit path and the I/O d.c. offset of the MRX to be estimated.

In accordance with aspects of this invention, there is provided anapparatus for estimating a d.c. offset in a transmitter having atransmit path for quadrature modulating a carrier with input I and Qcomponent signals and generating a transmit signal. The apparatusincludes a measurement receiver, a phase shifter, and an offsetestimator. The measurement receiver is configured to quadraturedemodulate a portion of the transmit signal to generate an I componentmeasurement signal and a Q component measurement signal. The phaseshifter is configured to generate a first pair of oscillator signalshaving a relative phase shift of substantially 90 degrees for quadraturedemodulation in the measurement receiver and for quadrature modulationin the transmit path. The phase shifter is also configured toselectively generate a second pair of oscillator signals having arelative phase shift of substantially 90 degrees and a phase shift of180 degrees with respect to the first pair of oscillator signals forquadrature demodulation in the measurement receiver. The offsetestimator is configured to compute at least one of a d.c. offset of thetransmit path and a d.c. offset of the measurement receiver based on theinput I and Q component signals and on measurement I and Q componentsignals generated with the first and second pairs of oscillator signals.

In other aspects, there is provided a method of estimating a d.c. offsetin a transmitter having a transmit signal generated by quadrature mixinginput I and Q component signals with respective ones of a first pair ofoscillator signals having a relative phase shift of substantially 90degrees. The method includes generating a first pair of measurement Icomponent and Q component measurement signals by quadrature demodulatinga portion of the transmit signal with the first pair of oscillatorsignals; generating a second pair of measurement I component and Qcomponent signals by quadrature demodulating a portion of the transmitsignal with a second pair of oscillator signals having a relative phaseshift of substantially 90 degrees and a relative phase shift withrespect to the first pair of oscillator signals of substantially 180degrees; and computing the d.c. offset based on the first and secondpairs of measurement I and Q component signals and on the input I and Qcomponent signals.

In other aspects, there is provided a computer-readable medium havingstored instructions that, when executed by a computer, cause thecomputer to perform a method of estimating a d.c. offset in atransmitter having a transmit signal generated by quadrature mixinginput I and Q component signals with respective ones of a first pair ofoscillator signals having a relative phase shift of substantially 90degrees. The method includes generating a first pair of measurement Icomponent and Q component measurement signals by quadrature demodulatinga portion of the transmit signal with the first pair of oscillatorsignals; generating a second pair of measurement I component and Qcomponent signals by quadrature demodulating a portion of the transmitsignal with a second pair of oscillator signals having a relative phaseshift of substantially 90 degrees and a relative phase shift withrespect to the first pair of oscillator signals of substantially 180degrees; and computing the d.c. offset based on the first and secondpairs of measurement I and Q component signals and on the input I and Qcomponent signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The several objects, features, and advantages of this invention will beunderstood by reading this description in conjunction with the drawings,in which:

FIG. 1 is a block diagram of transmitter with a measurement receiver;

FIG. 2 is a block diagram of a portion of an improved transmitter with ameasurement receiver;

FIG. 3 is a flow chart of method of d.c. offset estimation;

FIG. 4 depicts a communication network; and

FIG. 5 is a block diagram of a user equipment for the communicationnetwork.

DETAILED DESCRIPTION

This invention is applicable to any type of communication system and canbe applied in any part of the system, e.g., uplink (UL) or downlink(DL), where d.c.-offset estimation is of interest.

FIG. 2 is a block diagram of a portion of an improved transmitter 200with a measurement receiver in accordance with this invention. Thetransmitter 200 includes a coupler 202, a transmit path that has ananalog part 204 and a digital part 206, a voltage-controlled oscillator(VCO) 208, a phase shifter 210, an MRX 212, and an offset estimator 214.

The digital part 206 of the transmit path includes a digital transmitsignal waveform generator (WFG) 216 that produces an in-phase transmitsignal i_(tx) and a quadrature transmit signal q_(tx), which areconverted to analog form by respective digital-to-analog converters(DACs). The analog part 204 of the transmit path uses the analogtransmit signal components produced by the DACs to quadrature-modulate acarrier signal generated by the VCO 208 or equivalent local oscillator(LO) in the usual way with two mixers fed respectively by an unshiftedand 90-degree shifted LO signal. The phase shifter 210 implements the0-degree and 90-degree phase shifts of the LO signal needed for thequadrature modulation in the transmit path, and for quadraturedemodulation in the MRX 212 as described below. The modulated carrier issuitably amplified, for example by a power amplifier PA andvariable-gain amplifier VGA in the analog part 204, and the complextransmitter output signal is passed to an antenna or other port (notshown) through the coupler 202.

The waveform generator 216 also provides an in-phase reference signali_(ref) and a quadrature reference signal q_(ref) to the estimator 214.The reference signals i_(ref) and ref are simply the transmit signalsi_(tx) and q_(tx), respectively, and are used by the estimator 214 asdescribed in more detail below. In general, the generator 216 generatessignals that are compliant with the applicable modulation type andsystem standards in terms of data rate, pulse shaping filter, data (IQ)constellation, etc. The artisan will understand that the generator 216can be implemented in a substantially conventional way, although arelevant aspect of the generator 216 is the time alignment between thesignal that is sent (i_(tx) and q_(tx)) and the signal that is measured(i_(ref) and q_(ref)). In order to ensure a suitable time alignment, thegenerator 216 can include or implement a suitable variable delayelement, or such a delay element can be provided elsewhere in thetransmitter 200, to adjust the reference signals with respect to thetransmit signal.

As depicted in FIG. 2, the MRX 212 receives from the coupler 202 aportion of the transmit signal generated by the transmit path. Thatportion is amplified by a suitable low-noise amplifier LNA and passed toa quadrature demodulator that includes two mixers fed by selectivelyphase-shifted LO signals from the phase shifter 210. The down-shifted(demodulated) I and Q component signals produced by respective ones ofthe mixers are low-pass filtered and converted to digital componentmeasurement signals i_(meas) and q_(meas) by respective suitableanalog-to-digital converters ADC.

In addition to nominal 0-degree and nominal 90-degree phase shifts usedfor the quadrature modulator in the analog part 204 and for thequadrature demodulator in the MRX 212, the phase shifter 210 alsogenerates nominal 180-degree and nominal 270-degree phase shifts of theLO signal for the quadrature demodulator in the MRX 212. The MRX 212performs a first measurement with the “normal” phase shift, i.e., 0degrees and 90 degrees for the I and Q components, respectively, in theMRX quadrature demodulator, and a second measurement with a “special”phase-shift, i.e., 180 degrees and 270 degrees for the I and Qcomponents, respectively.

It is currently preferred that the phase shifter 210 generates theshifts in successive pairs for particular time intervals, but it will benoted that the phase shifter 210 can be configured to provide fourcontinuous outputs to the MRX 212, which could then have two paralleldemodulators, each comprising two mixers, generating four continuousmeasurement signal components. Although such an arrangement has someadvantages, it requires extra physical space, more power, and carefulmatching of the pairs of mixers.

In general, using the MRX 212 as little as possible is desirable inorder to save power, which can be important in a battery-poweredtransmitter. It is currently expected that the MRX would be used forperiods of 25-50 microseconds, with an approximately 50% duty cycle ofthe 0/180 and 90/270 phase shifts, which of course are synchronized tothe transmit path because the transmit and MRX measurement signals haveto be time-aligned as noted above. It is also currently expected thatthe noisiness of the MRX measurement signals should be reduced bysuitable smoothing, for example by low-pass filters, resettableintegrators, or simply averaging. It will be noted that the equationsgiven below are in terms of discrete signal samples, and do not includesuch smoothing, which can be implemented in many suitable ways, forexample by software programming in the offset estimator 214.

The combination of the phase shifter 210, MRX 212, and offset estimator214 as described above enables the d.c. offset in the transmit path,comprising the analog and digital parts 204, 206, to be distinguishedfrom the d.c. offset in the MRX 212. The estimator 214, which may be asuitably programmed digital processor or collection of logic gates, cancompute either or both of the d.c. offsets according to the followingequations and provide the computed offsets as results that can be usedby other components in the transmitter 200.

As depicted in FIG. 2, an offset result generated by the estimator 214,which is an estimate of the d.c. offset, can be fed back to manipulateeither the analog or digital parts 204, 206 or the MRX 212. For example,the offset result can be fed back to the transmit digital part 206 byproviding the result to the transmit generator 216, which can thencompensate the transmit signals i_(tx), q_(tx) it generates based on theoffset result. Such compensation can include simply adding the offsetresult to the transmit signals either in the generator 216 by includingsuitable adders (not shown for clarity) external to the generator 216.For another example, the offset result can be combined with the analogtransmit signals in the analog part 204 by suitable adders includedbefore the mixers of the quadrature modulator. An offset resultgenerated by the estimator 214 can also or instead be fed back to theMRX 212 as depicted in FIG. 2 through suitable adders included after themixers in the quadrature demodulator. Also as depicted in FIG. 2, theoffset result generated by the estimator 214 can be provided to othercomponents (not shown) in the transmitter 200, for example, forinclusion in a report message.

Thus, FIG. 3 is a flow chart of a method of estimating a d.c. offset ina transmitter having a transmit signal generated by quadrature mixinginput I and Q component signals with respective ones of a first pair ofoscillator signals having a relative phase shift of substantially 90degrees, e.g., 0-degree and 90-degree signals coming from the phaseshifter 210. In step 302, a first pair of measurement I component and Qcomponent measurement signals, such as i_(meas) and q_(meas) describedabove, is generated by quadrature demodulating a portion of the transmitsignal with the first pair of oscillator signals. In step 304, a secondpair of measurement I component and Q component signals by quadraturedemodulating a portion of the transmit signal with a second pair ofoscillator signals having a relative phase shift of substantially 90degrees and a relative phase shift with respect to the first pair ofoscillator signals of substantially 180 degrees, e.g., 180-degree and270-degree signals from the phase shifter 210. In step 306, the d.c.offset is computed based on the first and second pairs of measurement Iand Q component signals and on the input I and Q component signals.

The complex transmitter output signal including d.c. offset of thetransmit path can be written as follows:

z _(TX) =i _(ref) +q _(ref) +z _(DC,TX) =i _(ref) +q _(ref) +i _(DC,TX)+q _(DC,TX)  Eq. 1

in which z_(TX) is the transmitter output signal, i_(ref) is an Ichannel reference, q_(ref) is a Q channel reference, and z_(DC,TX) isthe d.c. offset of the transmit path, which can be separated as showninto d.c. offsets of the I and Q components of the transmit path,i_(DC,TX) and q_(DC,TX), respectively.

In a similar way, the complex output signal including d.c. offset of theMRX 212 can be written as follows:

z _(MRX) =i _(meas) +q _(meas)  Eq. 2

in which z_(MRX) is the MRX output signal, i_(meas) is the I channelsignal measured by the MRX 212, and q_(meas) is the Q channel signalmeasured by the MRX 212. The combined d.c. offset z_(DC,tot) of thetransmit path and the MRX 212 can be written as follows:

z _(DC,tot) =Z _(DC,TX) +Z _(DC,MRX) =i _(DC,TX) +q _(DC,TX) +i_(DC,MRX) +q _(DC,MRX)  Eq. 3

in which z_(DC,MRX) is the d.c. offset of the MRX 212 and the otherparameters are as defined above.

With the usual 0-degree and 90-degree phase shifts in the transmit pathand MRX 212, the 1-channel and Q-channel measurement signals generatedby the MRX 212 can be written as follows:

i _(meas) =i _(ref) +i _(DC,MRX) +i _(DC,TX)  Eq. 4

q _(meas) =q _(ref) +q _(DC,MRX) +q _(DC,TX)  Eq. 5

and with the “special” 180-degree and 270-degree phase shifts in the MRX212, the I-channel and Q-channel measurement signals generated by theMRX 212 can be written as follows:

î _(meas) =−i _(ref) +i _(DC,MRX) −i _(DC,TX)  Eq. 6

{circumflex over (q)} _(meas) =−q _(ref) +q _(DC,MRX) −q _(DC,TX)  Eq. 7

in which the “hat” indicates the “special” phase shifts.

Adding Eq. 4 and Eq. 6 yields the following:

i _(meas) +î _(meas)=2i _(DC,MRX)  Eq. 8A

which can be re-arranged to give the 1-channel d.c. offset of the MRX212 as follows:

$\begin{matrix}{i_{{D\; C},{MRX}} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}} & {{{Eq}.\mspace{14mu} 8}B}\end{matrix}$

In a similar way, adding Eq. 5 and Eq. 7 yields the following:

q _(meas) +{circumflex over (q)} _(meas)=2q _(DC,MRX)  Eq. 9A

which can be re-arranged to give the Q-channel d.c. offset of the MRX212 as follows:

$\begin{matrix}{q_{{D\; C},{MRX}} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}} & {{{Eq}.\mspace{14mu} 9}B}\end{matrix}$

Subtracting Eq. 6 from Eq. 4 yields the following:

i _(meas) −î _(meas)=2i _(ref)+2i _(DC,TX)  Eq. 10A

which can be re-arranged to give the I-channel d.c. offset of thetransmit path as follows:

$\begin{matrix}{i_{{D\; C},{TX}} = \frac{i_{meas} - {\overset{}{i}}_{meas} - {2i_{ref}}}{2}} & {{{Eq}.\mspace{14mu} 10}B}\end{matrix}$

In a similar way, subtracting Eq. 7 from Eq. 5 yields the following:

q _(meas) −{circumflex over (q)} _(meas)=2q _(ref)+2q _(DC,TX)  Eq. 11A

which can be re-arranged to give the Q-channel d.c. offset of thetransmit path as follows:

$\begin{matrix}{q_{{D\; C},{TX}} = \frac{q_{meas} - {\overset{}{q}}_{meas} - {2q_{ref}}}{2}} & {{{Eq}.\mspace{14mu} 11}B}\end{matrix}$

The offset estimator 214 can compute the individual d.c. offsets of thetransmit path and the MRX 212 using Eqs. 8B, 9B, 10B, and 11B.

The estimator 214 and various other functional blocks of the transmitter200 can be implemented by one or more suitably programmed electronicprocessors, collections of logic gates, etc. that process informationstored in one or more memories. The stored information may includeprogram instructions and data that enable the estimator 214 to implementthe equations described above.

FIG. 4 is a diagram of an exemplary communication network 400, which maybe, for example, a WCDMA communication system. Radio network controllers(RNCs) 402 a, 402 b control various radio network functions, includingfor example radio access bearer setup, diversity handover, etc. Moregenerally, each RNC directs user calls via the appropriate RBSs, whichcommunicate with user equipments (UEs) 300 a, 300 b through downlink(i.e., base-to-mobile, or forward) and uplink (i.e., mobile-to-base, orreverse) channels. RNC 402 a is shown coupled to RBSs 404 a, 404 b, 404c, and RNC 402 b is shown coupled to RBSs 404 d, 404 e, 404 f. Each RBS,which can also be called a NodeB, serves a geographical area that can bedivided into one or more cell(s). RBS 404 f is shown as having fiveantenna sectors S1-S5, all or some of which can be said to make up thecell of the RBS 404 f. The RBSs are coupled to their corresponding RNCsby dedicated telephone lines, optical fiber links, microwave links, etc.Both RNCs 402 a, 402 b are typically connected with external networkssuch as the public switched telephone network (PSTN), the Internet, etc.through one or more core network nodes, such as a mobile switchingcenter and/or a packet radio service node (not shown). The artisan willunderstand that the components and arrangement depicted in FIG. 4 areexamples and should not be construed as limiting the components andarrangement of an actual communication system.

FIG. 5 depicts a communication device 500, such as a mobile telephone,remote terminal, or equivalent device, that can communicate through awireless link with a base station in a communication network. The device500 can be a UE 300 in the network 400. Among other things, the UE 500includes one or more programmable processors 502 or suitable logic thatprocesses information stored in one or more memories 504, 506. Thestored information may include, among other things, program instructionsfor computing the d.c. offsets as described above. It will beappreciated that the processor 502 typically includes timers, etc. thatfacilitate its operations. Transceiver (TRX) circuitry 508 provides forthe reception and transmission of control and traffic signals on thelink between the UE 500 and the base station, which can include similartransceiver circuitry. The TRX 508 includes the transmitter portion 200described above that operates under the control of the processor 502.

It is expected that this invention can be implemented in a wide varietyof environments, including for example mobile communication devices. Itwill be appreciated that procedures described above are carried outrepetitively as necessary. To facilitate understanding, many aspects ofthe invention are described in terms of sequences of actions that can beperformed by, for example, elements of a programmable computer system.It will be recognized that various actions could be performed byspecialized circuits (e.g., discrete logic gates interconnected toperform a specialized function or application-specific integratedcircuits), by program instructions executed by one or more processors,or by a combination of both. Many communication devices can easily carryout the computations and determinations described here with theirprogrammable processors and application-specific integrated circuits.

Moreover, the invention described here can additionally be considered tobe embodied entirely within any form of computer-readable storage mediumhaving stored therein an appropriate set of instructions for use by orin connection with an instruction-execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that can fetch instructions from a medium and execute theinstructions. As used here, a “computer-readable medium” can be anymeans that can contain, store, or transport the program for use by or inconnection with the instruction-execution system, apparatus, or device.The computer-readable medium can be, for example but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device. More specific examples (anon-exhaustive list) of the computer-readable medium include anelectrical connection having one or more wires, a portable computerdiskette, a RAM, a ROM, an erasable programmable read-only memory (EPROMor Flash memory), and an optical fiber.

Thus, the invention may be embodied in many different forms, not all ofwhich are described above, and all such forms are contemplated to bewithin the scope of the invention. For each of the various aspects ofthe invention, any such form may be referred to as “logic configured to”perform a described action, or alternatively as “logic that” performs adescribed action.

It is emphasized that the terms “comprises” and “comprising”, when usedin this application, specify the presence of stated features, integers,steps, or components and do not preclude the presence or addition of oneor more other features, integers, steps, components, or groups thereof.

The particular embodiments described above are merely illustrative andshould not be considered restrictive in any way. The scope of theinvention is determined by the following claims, and all variations andequivalents that fall within the range of the claims are intended to beembraced therein.

1. An apparatus for estimating a direct-current (d.c.) offset in atransmitter having a transmit path for quadrature modulating a carrierwith input in-phase (I) and quadrature (Q) component signals andgenerating a transmit signal, the apparatus comprising: a measurementreceiver, wherein the measurement receiver is configured to quadraturedemodulate a portion of the transmit signal to generate an I componentmeasurement signal and a Q component measurement signal; a phaseshifter, wherein the phase shifter is configured to generate a firstpair of oscillator signals having a relative phase shift ofsubstantially 90 degrees for quadrature demodulation in the measurementreceiver and for quadrature modulation in the transmit path, and thephase shifter is configured to selectively generate a second pair ofoscillator signals having a relative phase shift of substantially 90degrees and a phase shift of substantially 180 degrees with respect tothe first pair of oscillator signals for quadrature demodulation in themeasurement receiver; and an offset estimator, wherein the offsetestimator is configured to compute at least one of a d.c. offset of thetransmit path and a d.c. offset of the measurement receiver based on theinput I and Q component signals and on measurement I and Q componentsignals generated with the first and second pairs of oscillator signals.2. The apparatus of claim 1, wherein the offset estimator is configuredto compute an I component d.c. offset of the measurement receiveraccording to:$i_{{D\; C},{MRX}} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}$ inwhich i_(DC,MRX) is the I component of the d.c. offset of themeasurement receiver, i_(meas) is the measurement I component signalgenerated with one of the first pair of oscillator signals, and i_(meas)is the measurement I component signal generated with one of the secondpair of oscillator signals having a 180-degree phase shift with respectto the one of the first pair of oscillator signals; and the offsetestimator is configured to compute a Q component d.c. offset of themeasurement receiver according to:$q_{{D\; C},{MRX}} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}$ inwhich q_(DC,MRX) is the Q component of the d.c. offset of themeasurement receiver, q_(meas) is the measurement Q component signalgenerated with the other one of the first pair of oscillator signals,and {circumflex over (q)}_(meas) is the measurement Q component signalgenerated with the other one of the second pair of oscillator signals.3. The apparatus of claim 1, wherein the offset estimator is configuredto compute an I component d.c. offset of the transmit path according to:$i_{{D\; C},{TX}} = \frac{i_{meas} - {\overset{}{i}}_{meas} - {2i_{ref}}}{2}$in which i_(DC,TX) is the I component of the d.c. offset of the transmitpath, i_(meas) is the measurement I component signal generated with oneof the first pair of oscillator signals, î_(meas) is the measurement Icomponent signal generated with one of the second pair of oscillatorsignals having a 180-degree phase shift with respect to the one of thefirst pair of oscillator signals, and i_(ref) is the I component signal;and the offset estimator is configured to compute a Q component d.c.offset of the transmit path according to:$q_{{D\; C},{TX}} = \frac{q_{meas} - {\overset{}{q}}_{meas} - {2q_{ref}}}{2}$in which q_(DC,TX) is the Q component of the d.c. offset of the transmitpath, q_(meas) is the Q component measurement signal generated with theother one of the first pair of oscillator signals, {circumflex over(q)}_(meas) is the measurement Q component signal generated with theother one of the second pair of oscillator signals, and q_(ref) is the Qcomponent signal.
 4. A method of estimating a direct-current (d.c.)offset in a transmitter having a transmit signal generated by quadraturemixing input in-phase (I) and quadrature (Q) component signals withrespective ones of a first pair of oscillator signals having a relativephase shift of substantially 90 degrees, the method comprising:generating a first pair of measurement I component and Q componentmeasurement signals by quadrature demodulating a portion of the transmitsignal with the first pair of oscillator signals; generating a secondpair of measurement I component and Q component signals by quadraturedemodulating a portion of the transmit signal with a second pair ofoscillator signals having a relative phase shift of substantially 90degrees and a relative phase shift with respect to the first pair ofoscillator signals of substantially 180 degrees; and computing the d.c.offset based on the first and second pairs of measurement I and Qcomponent signals and on the input I and Q component signals.
 5. Themethod of claim 4, wherein computing the d.c. offset includes computingan I component d.c. offset according to:$i_{{D\; C},{MRX}} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}$ inwhich i_(DC,MRX) is the I component d.c. offset, i_(meas) is themeasurement I component signal generated with one of the first pair ofoscillator signals, and î_(meas) is the measurement I component signalgenerated with one of the second pair of oscillator signals having a180-degree phase shift with respect to the one of the first pair ofoscillator signals; and computing the d.c. offset includes computing a Qcomponent d.c. offset according to:$q_{{D\; C},{MRX}} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}$ inwhich q_(DC,MRX) is the Q component d.c. offset, q_(meas) is themeasurement Q component signal generated with the other one of the firstpair of oscillator signals, and {circumflex over (q)}_(meas) is themeasurement Q component signal generated with the other one of thesecond pair of oscillator signals.
 6. The method of claim 4, whereincomputing the d.c. offset includes computing an I component d.c. offsetof the transmit path according to:$i_{{D\; C},{TX}} = \frac{i_{meas} - {\overset{}{i}}_{meas} - {2i_{ref}}}{2}$in which i_(DC,TX) is the I component of the d.c. offset of the transmitpath, i_(meas) is the measurement I component signal generated with oneof the first pair of oscillator signals, î_(meas) is the measurement Icomponent signal generated with one of the second pair of oscillatorsignals having a 180-degree phase shift with respect to the one of thefirst pair of oscillator signals, and i_(ref) is the I component signal;and computing the d.c. offset includes computing a Q component d.c.offset of the transmit path according to:$q_{{D\; C},{TX}} = \frac{q_{meas} - {\overset{}{q}}_{meas} - {2q_{ref}}}{2}$in which q_(DC,TX) is the Q component of the d.c. offset of the transmitpath, q_(meas) is the Q component measurement signal generated with theother one of the first pair of oscillator signals, {circumflex over(q)}_(meas) is the measurement Q component signal generated with theother one of the second pair of oscillator signals, and q_(ref) is the Qcomponent signal.
 7. A computer-readable medium having storedinstructions that, when executed by a computer, cause the computer toperform a method of estimating a direct-current (d.c.) offset in atransmitter having a transmit signal generated by quadrature mixinginput in-phase (I) and quadrature (Q) component signals with respectiveones of a first pair of oscillator signals having a relative phase shiftof substantially 90 degrees, wherein the method comprises: generating afirst pair of measurement I component and Q component measurementsignals by quadrature demodulating a portion of the transmit signal withthe first pair of oscillator signals; generating a second pair ofmeasurement I component and Q component signals by quadraturedemodulating a portion of the transmit signal with a second pair ofoscillator signals having a relative phase shift of substantially 90degrees and a relative phase shift with respect to the first pair ofoscillator signals of substantially 180 degrees; and computing the d.c.offset based on the first and second pairs of measurement I and Qcomponent signals and on the input I and Q component signals.
 8. Themedium of claim 7, wherein computing the d.c. offset includes computingan I component d.c. offset according to:$i_{{D\; C},{MRX}} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}$ inwhich i_(DC,MRX) is the I component d.c. offset, i_(meas) is themeasurement I component signal generated with one of the first pair ofoscillator signals, and î_(meas) is the measurement I component signalgenerated with one of the second pair of oscillator signals having a180-degree phase shift with respect to the one of the first pair ofoscillator signals; and computing the d.c. offset includes computing a Qcomponent d.c. offset according to:$q_{{D\; C},{MRX}} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}$ inwhich q_(DC,MRX) is the Q component d.c. offset, q_(meas) is themeasurement Q component signal generated with the other one of the firstpair of oscillator signals, and {circumflex over (q)}_(meas) is themeasurement Q component signal generated with the other one of thesecond pair of oscillator signals.
 9. The medium of claim 7, whereincomputing the d.c. offset includes computing an I component d.c. offsetof the transmit path according to:$i_{{D\; C},{TX}} = \frac{i_{meas} - {\overset{}{i}}_{meas} - {2i_{ref}}}{2}$in which i_(DC,TX) is the I component of the d.c. offset of the transmitpath, i_(meas) is the measurement I component signal generated with oneof the first pair of oscillator signals, î_(meas) is the measurement Icomponent signal generated with one of the second pair of oscillatorsignals having a 180-degree phase shift with respect to the one of thefirst pair of oscillator signals, and i_(ref) is the I component signal;and computing the d.c. offset includes computing a Q component d.c.offset of the transmit path according to:$q_{{D\; C},{TX}} = \frac{q_{meas} - {\overset{}{q}}_{meas} - {2q_{ref}}}{2}$in which q_(DC,TX) is the Q component of the d.c. offset of the transmitpath, q_(meas) is the Q component measurement signal generated with theother one of the first pair of oscillator signals, {circumflex over(q)}_(meas) is the measurement Q component signal generated with theother one of the second pair of oscillator signals, and q_(ref) is the Qcomponent signal.